Field of the Invention
The invention relates to a recording material for a semiconductor memory, and more particularly, to a multi-layer phase change material having low thermal conductivity.
Description of the Related Art
Typical memories are divided into three types: magnetic memories, optical memories, and semiconductor memories, amongst, the semiconductor memories are widely used because of their excellent performance. Main semiconductor memories that are currently used are flash disks based on a floating gate structure. However, due to the thickness of the floating gate, semiconductor memories cannot break through the bottleneck of a size of 32 nm with the further development of IC (Integrated Circuit) technology, people now begin to competitively develop the next-generation memories represented by phase change random access memories (PCRAM).
A PCRAM is a nonvolatile semiconductor memory utilizing heat effect of electric pulses to facilitate reversible change of a recording material between a crystalline state and an amorphous state, thus storing data via big difference in resistance values under the two states. The recording material is made of film phase change materials mainly including chalcogenide compounds, a crystalline state thereof is a low resistance state representing a data bit “1”, and an amorphous state thereof is a high resistance state representing a data bit “0”. Temperature required for changing a phase change material from the amorphous state to the crystalline state is referred to as crystallization temperature, and temperature required for changing the phase change material from the crystalline state to the amorphous state is referred to as melting temperature.
Since advent of the PCRAM, it has received much concern due to its good performance in no volatility, compatibility with the CMOS technique, high-speed, resistance to radiation, low-price, and long service life. The Semiconductor Industry Association regards the PCRAM as a mainstream memory product in the future capable of replacing existing products such as flash disks, DRAMs and so on. However, with reduction in size of the PCRAM, distance between adjacent memory cells also decreases. Upon reading or writing a memory cell, heat generated resulting therefrom is inevitably transferred to adjacent memory cells. Once temperature rise of the adjacent memory cells caused by heat transfer exceeds phase change temperature of a recording material, unintended variation of a recording state of the memory cell will occur, and heat interference between adjacent memory cells will greatly affect reliability of the memory. Moreover, melting temperature of existing recording materials is comparatively high, and thus thermal energy required for phase change thereof is comparatively high. As a result, large power consumption becomes another big bottleneck that restricts further application of the PCRAM.
To improve reliability of the PCRAM, a common-used method is to mix elements such as N, O, Sn and so on into the film phase change material, thereby increasing crystallization temperature of a recording material and preventing errors of data bits caused by temperature rise of adjacent memory cells exceeding phase change temperature of recording materials thereof. Although temperature rise does not result in fast change of a state of the recording material, the method does not reduce heat transferred to adjacent memory cells, a resistance value of the recording material is significantly changed, and data bits are easy to fail after many times of operation, and thus a work life of the memory is greatly affected. Nevertheless, as the crystallization temperature is raised, melting temperature of the film phase change material is correspondingly increased, which results in higher power consumption of the PCRAM.
To summarize, it is very urgent to provide a phase change material with good thermal performance that is capable of reducing heat interference between adjacent memory cells of the PCRAM and power consumption thereof.